So I missed the crossover…
In summary of our confusion…
Tasha made the Demonomicons. The Shards of the Sundering are within these books. We aren’t sure how they got there or how involved Tasha/Iggwilb was in putting them there. These shards make up a Shard of Pure Evil that was within the Black Diamond. The diamond was mined by Dwarves within the feywild. They meant to gift the diamond to the Summer Queen, but she was swimming and her sister, who’s name has been lost or purposefully forgotten, received it having disguised herself as the summer queen. Somehow it was thrown at the material plane creating the rift and the sundering splitting the world and into the 6 Shards of the Sundering.
Remish got us to Rootwallow. The thing above it is filled with modrons.